1. Field of the Invention
The present invention relates to an optical switch and an optical demultiplexer, and more particularly to an optical switch and an optical demultiplexer which are simplified in structure and control.
2. Description of the Related Art
Recently, a wavelength division multiplexing (WDM) optical communication system has been developed as a broadband optical communication system. Other optical communication systems, such as optical time division multiplexing (OTDM) and time wavelength division multiplexing (TWDM), have also been proposed and studied aiming at broader band optical communication.
The WDM optical communication system is a system for increasing signal density through wavelength multiplexing of an optical signal. The time division systems, such as OTDM and TWDM optical communication systems, are intended to increase signal density by time-dividing an optical signal of the same wavelength and assigning divided optical signals to a number of channels.
Response speed of an electrical signal is limited by a moving time of carriers in a semiconductor device and hence lower than the response speed of an optical signal. At present, the speed limit of an electrical signal is thought to be about 40 Gbits/s. To process an OTDM signal having speed higher than that limit, an optical signal must be divided through high-speed optical signal processing and demultiplexed to a bit rate, at which electrical processing is feasible.
In view of the above-mentioned background, an optical device (optical demultiplexer) has recently been studied which is able to demultiplex an optical signal, as it is, without converting the optical signal into an electrical signal. Hitherto, optical demultiplexers of, e.g., non-linear optical loop mirror (NOLM) type, Mach-Zehnder type and polarization separating type, have been proposed.
FIG. 15A is a schematic view of a NOLM type optical demultiplexer. An optical signal sig1 reaches a branch point 102 of an optical fiber loop 101 via an input side optical fiber 100. At the branch point 102, the optical signal sig1 is branched into an optical signal sig2 propagating in the loop 101 counterclockwise and an optical signal sig3 propagating in the loop 101 clockwise. The optical signal sig1 is a signal having four time-division multiplexed channels, i.e., channels #1 to #4.
A non-linear waveguide 103 is inserted in the optical loop 101 at a position asymmetrical to the branch point 102. The optical signal sig2 propagating counterclockwise reaches the non-linear waveguide 103 at timing earlier than the optical signal sig3 propagating clockwise. A control light pulse con is inputted to the non-linear waveguide 103 immediately after the channel #2 of the optical signal sig2 has passed the non-linear waveguide 103. The refractive index of the non-linear waveguide 103 is changed upon the inputting of the control light pulse con, whereby the phase of a pulse light in each channel #3 and #4 of the optical signal sig2 is shifted π. In FIG. 15A, a pulse having phase shifted π is represented by hatching.
Because the optical signal sig3 reaches the non-linear waveguide 103 at timing delayed from the optical signal sig2, only the channel #1 of the optical signal sig3 has passed the non-linear waveguide 103 at the time when the control light pulse con is inputted to the non-linear waveguide 103. Therefore, the phase of a pulse light in each of the channels #2 to #4 of the optical signal sig3 is shifted π.
When the optical signals sig2 and sig3 return to the branch point 102, the pulses in those ones #1, #3 and #4 of the channels of both the signals, which are in phase, propagate in the input side optical fiber 100, and the pulse in the out-of-phase channel #2 propagates in an output side optical fiber 105. Thus, only the signal of one channel can be separated from the time division multiplexed signal sig1.
In the NOLM type optical demultiplexer, the time required for the optical signal to pass the optical loop 101 limits the signal speed achievable in signal processing. Also, the use of an optical fiber loop raises a difficulty in reducing the device size.
FIG. 15B is a schematic view of a Mach-Zehnder type optical demultiplexer. Non-linear waveguides 121 and 122 are inserted respectively in two arms of a Mach-Zehnder interferometer 120. An optical signal sig10 is branched into two optical signals sig11 and sig12, which are introduced to the non-linear waveguides 121 and 122, respectively. A control light pulse con is inputted to the non-linear waveguides 121 and 122 at different timings from each other.
The control light pulse con is inputted to the non-linear waveguide 121 immediately after a pulse in a channel #1 has passed the non-linear waveguide 121, and is inputted to the non-linear waveguide 122 immediately after a pulse in a channel #2 has passed the non-linear waveguide 122. Therefore, the phase of an optical pulse in each of the channels #2 to #4 of the optical signal sig11 is shifted π after passing the non-linear waveguide 121, and the phase of an optical pulse in each channel #3 and #4 of the optical signal sig12 is shifted π after passing the non-linear waveguide 122.
When the optical signals sig11 and sig12 are combined with each other, the signals in the channels #1, #3 and #4 are introduced to one output optical fiber 125, and the signal in the channel #2 is introduced to the other output optical fiber 126.
Thus, in the Mach-Zehnder type optical demultiplexer, two arms, in which non-linear waveguides are respectively inserted, must be arranged parallel to each other. The device size is therefore increased.
FIG. 15C is a schematic view of a polarization separating type optical demultiplexer. An optical signal sig20 enters a birefringence crystal 130. The birefringence crystal 130 delays a light in the TM mode by one pulse relative to a light in the TE mode. An optical signal sig21 having passed the birefringence crystal 130 and a control light pulse con are both inputted to a non-linear waveguide 131. The control light pulse con is inputted to the non-linear waveguide 131 immediately after a TE-mode pulse in the channel #2 has passed the non-linear waveguide 131.
In an optical signal sig22 having passed the non-linear waveguide 131, therefore, the phase of the TE-mode optical pulse in each channel #3 and #4 is shifted π, and the phase of the TM-mode optical pulse in each of the channels #2 to #4 is shifted π. The optical signal sig22 having passed the non-linear waveguide 131 is inputted to another birefringence crystal 132. The birefringence crystal 132 delays a light in the TE mode by one pulse relative to a light in the TM mode. Accordingly, in an optical signal sig23 having passed the birefringence crystal 132, positions of the TM-mode pulses match respectively with positions of the TE-mode pulses in the corresponding channels.
In the optical signal Sig23, therefore, the TM-mode pulses and the TE-mode pulses are in phase in the channels #1, #3 and #4, but they have a phase difference therebetween in the channel #2. By introducing the optical signal sig23 to enter a polarizer 133, only the pulse of the channel #2 can be separated.
Thus, the polarization separating type optical demultiplexer is designed on condition that an inputted optical signal has intensities substantially equal to each other between the TM and TE modes. In general, however, the polarization state of an optical signal having propagated through an optical fiber is not constant. For that reason, the polarization separating type optical demultiplexer is not suitable for practical use.
As described above, the various types of conventional optical demultiplexers have problems such as a limitation in processing speed, an increased device size, and dependency on the polarization state of an optical signal.